The present invention relates to a copper-based, sintered sliding material provided with a steel back metal layer, particularly a copper-based, sintered sliding material in which the strength of the steel back metal layer and the bearing characteristics of a sintered layer are enhanced, and relates to a method of producing the same.
In prior arts, a conventional copper-based, sintered sliding material has been, in general, produced by a method comprising the steps of: spreading a Cu-based alloy powder on a steel back metal; performing the primary sintering of them in a reducing atmosphere at a temperature of 800 to 950xc2x0 C. for about 15 minutes; performing the primary rolling thereof so that a sintered layer may be compacted; performing the secondary sintering thereof under the same conditions as those of the primary sintering; and performing the secondary rolling thereof for the purposes of improving the strength and of adjusting the size thereof. Since this method is suitable for the mass production thereof and makes it possible to produce the sliding material at a low cost, the sliding material has been used to produce bearings used in the industries of a broad range.
Nowadays, there are such a tendency as a housing for mounting a bearing is made of a light alloy such as an aluminum alloy so as to meet the requirement of the light-weight-and-small-size design of a bearing device, and such another tendency as the bearing itself is made to have a thin thickness and a narrowed width for obtaining a low friction. On the other hand, the degree of fixing the bearing onto the housing depends on an interference (i.e. crush height), and it is necessary for the interference to have such a value as a radial stress applied onto the bearing is more than a predetermined level so as to prevent the bearing from being displaced in the housing while another stress occurring in the circumferential direction of the bearing due to the interference is not more than the yield point of the bearing (the steel back metal) so that the bearing may be prevented from being buckled.
However, as is often the case with a bearing device used in an internal combustion engine for a vehicle, there is such a case as the temperature range of the use of the bearing device is very large, for example, from a below-zero temperature (at the time of non-driving) to about 200xc2x0 C. (during driving). The inventors of the invention have found that, in this case, since the difference in thermal expansion between the housing made of the light alloy and the bearing becomes large, it is necessary to make the interference large so that the fixing of the bearing may not be loosened even at the high temperature. In the case of making the interference large, there occurs such a fear as, in a bearing having a thin thickness or having a narrow width, the circumferential stress becomes so large that the buckling thereof is caused. Once the buckling occurs, a clearance between a shaft and the bearing is decreased with the result that the seizure thereof is apt to occur, and/or the radial stress applied thereto is decreased with the result that the bearing comes to rotate together with the shaft or that the bearing comes to be separated from the housing or that fretting occurs between the inner face of the housing and the back face of the bearing. On the other hand, the inventors of the invention have also found that, in another case of making the interference small, the radial stress applied to the bearing becomes so small that the above problem such as the rotation of both of the bearing and shaft or the separation of the bearing or the fretting etc. occurs similarly to the case of the buckling.
In order to enhance the buckling resistance of the bearing by increasing the strength of the steel back metal layer, it is thought to make the rolling reduction of a secondary rolling large so that the strength of the steel back metal layer may be raised because of the work hardening thereof. However, in the case of making the rolling reduction large, each of the steel back metal layer and the sintered layer made of Cu or Cu alloy becomes hard in hardness and brittle, so that the bearing comes to crack during the press working applied thereto or becomes inferior in workability. Further, in this case, the sliding performances such as conformability and foreign matter embeddability etc. are deteriorated due to the hardening of the Cu or Cu alloy of which the sintered layer is made, so that the seizure thereof is apt to occur. In addition, the sliding material is subjected to a deformation during the secondary rolling, so that the metal grains thereof come to have much strain. In general, a metal material with strained grains causes a recovery phenomenon even at a temperature less than the recrystallization temperature thereof, so that the strain is relieved although the complete release thereof does not occur. Thus, in a bearing used in an internal combustion engine in which the temperature is raised up to about 200xc2x0 C., the recovery phenomenon is caused during the use thereof, due to which the circumferential length of the bearing is varied (reduced) when the strain put in the crystal grains at the secondary rolling is relieved, even at such a level of the interference as no buckling occurs at the time when the bearing is initially mounted in the housing so that the same problem as in the case where the bearing is buckled is caused. This problem occurs not only in the housing made of the light metal such as Al alloy but also in the housing made of a steel or a cast steel.
The invention is made in taking the above-explained circumstances of the prior art into consideration, and the first object of the invention is to provide a copper-based, sintered sliding material superior in sliding characteristics and workability in which the amount of decrease in the circumferential length of a bearing made of this sliding material is small when the bearing mounted in a housing is subjected to the use temperature thereof and which can keep a high yield point. The second object of the invention is to provide a method of producing the copper-based, sintered sliding material at a low cost.
According to the first aspect of the invention, there is provided a copper-based, sintered sliding material comprising a steel back metal layer and a sintered layer made of Cu or a Cu alloy which is bonded onto the steel back metal layer, the steel back metal layer having a hardness not less than 160 Hv and an elongation not less than 10%, the sintered layer having a hardness not more than 130 Hv and a grain size not more than 45 xcexcm.
In this copper-based, sintered sliding material having the above constitution, the sintered layer made of Cu or the Cu alloy has such a relatively soft structure as to be not more than 130 Hv in hardness, so that the sliding characteristics such as conformability and foreign matter embeddability are superior. Further, since the grain size of the Cu or Cu alloy is not more than 45 xcexcm, the sintered layer has a fine structure, so that the strength thereof becomes high although it is soft, with the result that the sintered layer becomes superior in fatigue resistance. The grain size used herein is measured in accordance with the method for estimating average grain size for wrought copper and copper alloy products which is defined in JIS H 0501.
On the other hand, since the steel back metal layer has the elongation not less than 10%, the strain in the crystal grains thereof is at a low level. Further, since the steel back metal layer has the hardness not less than 160 Hv, the yield point thereof becomes high, and the steel back metal layer can withstand a large circumferential stress when the sliding material is formed into a cylindrical bearing, so that no fear of the buckling of the bearing is caused. In addition, the steel back metal layer is previously subjected to a heat treatment for bringing about the recovery phenomenon thereof, there occurs no fear of decrease in the circumferential length of the bearing which is caused due to the relieving of the strains in the crystal grains during the recovery phenomenon. Further, since the sintered layer is soft with the elongation of the steel back metal layer being not less than 10%, the cracks of the sliding material hardly occurs even in the case where an impact force is applied thereto in the press working thereof, that is, the sliding material is superior in workability.
In the sintered layer, Sn (tin) of 1 to 11 mass % may be contained. Further, the sintered layer may contain P (phosphorus) not more than 0.2 mass %, or may contain Ni (nickel) and/or Ag (silver) not more than 10 mass % in total, or may contain Pb (lead) and/or Bi (bismuth) not more than 25 mass % in total.
The sintered layer may contain dispersed hard particles and/or the dispersed particles of a high melting point substance of not more than 15 volume % in total, or may contain a dispersed solid lubricant not more than 10 volume %. Further, on the surface of the sintered layer, a plating overlay layer may be provided which is made of a Pbxe2x80x94Sn alloy containing In and/or Cu.
The reasons for containing the above components are explained below.
(a) Sn: 1 to 11 mass %
Sn acts to enhance the strength of the Cu matrix, and acts, in the production method explained below, to lower the recrystallization temperature of the Cu alloy constituting the sintered layer and to enlarge a heat treatment temperature range (which is defined to be not less than the recrystallization-commencement temperature of Cu or the Cu alloy but less than the recrystallization-commencement temperature of the steel back metal layer). No effect is brought about in a case of the amount of Sn being less than 1 mass %, and the Cu alloy becomes brittle in another case of the amount of Cu being more than 11 mass %.
(b) P: not more than 0.2 mass %
P acts to enhance the strength of the Cu matrix. In the case where the amount of P exceeds 0.2 mass %, the sintered layer becomes too hard in hardness.
(c) Pb and/or Bi: not more than 25 mass % in total
Each of Pb and Bi acts to enhance the conformability of the Cu matrix. In the case where the amount of Pb and/or Bi exceeds 25 mass % in total, the sintered layer becomes brittle.
(d) Ni and/or Ag: not more than 10 mass % in total
Each of Ni and Ag acts to enhance the strength and corrosion resistance of the Cu matrix. In the case where the amount of Ni and/or Ag exceeds 10 mass % in total, the sintered layer becomes brittle.
(e) The hard particles and/or the particles of the high melting point substance: being dispersed by the amount not more than 15 volume % in total
The hard particles and/or the particles of the high melting point substance act to enhance the sliding characteristics of the sintered layer, the wear resistance thereof and the anti-seizure property thereof. In the case where the amount thereof exceeds 15 volume % in total, the sintered layer becomes brittle. The hard particles are ones of at least one kind selected from the group consisting of a metal oxide, a metal carbide, a metal silicide, and an intermetallic compound. The particles of the high melting point substance are ones of at least one kind selected from the group consisting of Mo (molybdenum), W (tungsten) and C (carbon).
(f) The solid lubricant: being dispersed by the amount not more than 10 volume %
The solid lubricant acts to enhance the lubrication of the sintered layer, however, it make the sintered layer brittle in the case where the amount of the solid lubricant exceeds 10 volume %. The solid lubricant is at least one kind selected from the group consisting of MoS2, WS2, BN and graphite.
The overlay layer acts to enhance the conformability of an initial stage, in which In acts to enhance the corrosion resistance and Cu acts to enhance the strength of the Pb matrix.
According to the second aspect of the invention, there is provided a method of producing the copper-based, sintered sliding material, comprising the steps of: spreading a powder of Cu or a Cu alloy on a steel sheet; performing a primary sintering thereof; performing a primary rolling thereof; performing a secondary sintering thereof; performing a secondary rolling thereof with a rolling reduction not less than 10%; and performing a heating treatment thereof at a temperature not less than the recrystallization-commencement temperature of Cu or the Cu alloy but less than the recrystallization-commencement temperature of a steel back metal layer.
In the primary rolling, sintering pores present in the sintering layer is crushed to make the sintering layer compacted. In the state of the primary sintering, the sintering layer has pores not less than 30 volume %, which make the sintered layer brittle. Thus, in a case of performing the primary rolling at a large rolling reduction, there occurs such shortcomings as the sintered layer is cracked, as the sintered layer is apt to be peeled off from the steel back metal layer and as strain is afforded unevenly to the sintered layer. Thus, it is preferred that the primary rolling is performed not with a large rolling reduction but with such a rolling reduction as the pores are crushed to make the sintered layer compacted. The pore portions present in the sintered layer which are crushed during the primary rolling do not come to be integrated through metallic bond, however, the pore portions comes to be integrated through the metallic bond during the secondary sintering.
In the secondary rolling after the secondary sintering, the rolling reduction not less than 10% is adopted so that the whole thickness of the copper-based, sintered sliding material becomes about 90% or less of the thickness thereof before the secondary rolling (, that is, a thickness after the secondary sintering). Even in the case where the secondary rolling is performed with this large rolling reduction not less than 10%, there occurs no fear of the cracking of the sintered layer or of the peeling-off thereof from the steel back metal layer because the sintered layer is made to have such a compacted structure as the pores present in the sintered layer are minimized by the primary rolling. Since the secondary rolling is performed with the rolling reduction not less than 10% with respect to the compacted sintered layer with minimized pores, it becomes possible to perform the rolling while affording a uniform strain over the whole of the sintered layer. By this rolling, the sintered layer comes to have an amorphous structure in which the crystal grains are broken finely, as shown in FIG. 5, and strains come to be present in the grains of the sintered layer and in the grains of the steel back metal layer.
In this state, the heating treatment is performed under the temperature condition not less than the recrystallization-commencement temperature of Cu or the Cu alloy but less than the recrystallization-commencement temperature of the steel back metal layer. In this case, since each of the recrystallization-commencement temperature of Cu or the Cu alloy and the recrystallization-commencement temperature of the steel constituting the steel back metal layer varies in dependence on the amount of the strains present in the crystal grains (, that is, the value of the rolling reduction of the secondary rolling), the temperature of the heating treatment is selected appropriately in accordance with the degree of the strains.
Because of this heat treatment, the recrystallization occurs in the sintered layer. In the recrystallization of the sintered layer, new crystal grains regenerated therein become uniform, fine grains each having a size not more than 45 xcexcm because the secondary rolling is performed with a large rolling reduction not less than 10% to thereby afford to the grains the uniform strains which become nuclei for the recrystallization and because the heating is performed at the temperature at which the grains of Cu or the Cu alloy hardly grow. The size of the grains is an arithmetic mean value calculated from the maximum grain size and minimum grain size of each of the grains.
In addition, the substantially uniform deformation is applied to the whole of the sintered layer through the secondary rolling performed after the secondary sintering, so that there does not occur such a fear as a part of the new crystal grains grows to become coarse grains due to the presence of locally small crystal strains. Thus, even in the case where the heating treatment is performed at a temperature somewhat exceeding the recrystallization-commencement temperature of Cu or the Cu alloy, or less than the recrystallization-commencement temperature of the steel back metal layer, the whole of the sintered layer comes to have a uniform, fine structure as shown in FIGS. 6 and 7.
Accordingly, by the heating treatment, Cu or the Cu alloy constituting the sintered layer is recrystallized, the strain therein being substantially completely released with Cu or the Cu alloy being softened, and the hardness thereof becomes not more than 130 Hv. However, since the new grains occurring by the recrystallization are fine in size, the sintered layer comes to have a high strength together with a high extensibility (workability).
On the other hand, since the heat treatment is performed at the temperature less than the recrystallization-commencement temperature of the steel back metal layer, no recrystallization occurs in the steel back metal layer. However, in the heat treatment, the recovery phenomenon occurs in the steel back metal layer, so that the crystal strains applied to the steel back metal layer by the secondary rolling is relieved. Since the relieving of the strains occurs not by the recrystallization thereof but by the recovery thereof, all of the strains are not released. Thus, the degree of decrease in the strength of the steel back metal layer, which has been worked to have a high strength by the secondary rolling, becomes small with the result that the steel back metal layer can keep a high hardness not less than 160 Hv and a high yield strength. Accordingly, when a half bearing or a cylindrical bearing made of the sliding material of the invention is attached onto a housing, no buckling of the bearing occurs even in a case of a large interference being adopted with respect to a bearing having a thin thickness or another bearing having a narrow width. Further, because of the relieving of the crystal strain, the thermal conductivity of the steel back metal layer becomes good, the elongation thereof becoming such a high level as to be not less than 10%, so that the extensibility (workability) thereof becomes good. Accordingly, there occurs no fear that the steel back metal layer is cracked during the press working.
Further, since the temperature of the heat treatment is made to be higher than that of the use of the bearing, the heat treatment acts to previously afford the recovery, which would occur during the use of the bearing if no heat treatment is performed, to the bearing. Thus, it becomes possible to prevent the variation (decrease) of the circumferential length of the bearing from being caused during the relieving of the strain due to the recovery phenomenon which occurs during the use of the bearing.
The time of the heating treatment is preferably 5 to 60 minutes. In a case of the time less than 5 minutes, both of the recovery phenomenon of the steel back metal layer and the recrystallization of the sintered layer become insufficient, however, in another case of the time exceeding 60 minutes, the crystal grains in the sintered layer become coarse in size, and this time makes the method unsuitable for the mass production using continuous sintering furnaces. In the production method of the invention, a further rolling applied to the material may be performed by use of rolls so as to adjust the dimensions of the sliding material after the heat treatment.
According to the third aspect of the invention, there is provided a sliding bearing adapted to be mounted in a housing, said bearing having, together with improved decrease in a circumferential length of said bearing, a superior resistance to buckling which is apt to occur when said sliding bearing is mounted in the housing with a large interference, said sliding bearing comprising a steel back metal layer, and a sintered layer made of Cu or a Cu-based alloy which is bonded onto the steel back metal layer, said steel back metal layer having a hardness not less than 160 Hv and an elongation not less than 10%, said sintered layer having a hardness not more than 130 Hv and crystal grains each provided with a grain size not more than 45 xcexcm.
In the sliding bearing, the sintered layer may be made of a Cu-based alloy containing 1 to 11 mass % Sn, a first optional element of P not more than 0.2 mass %, at least one second optional element not more than 10 mass % in total selected from the group consisting of Ni and Ag, and at least one third optional element not more than 25 mass % in total selected from the group consisting of Pb and Bi.
In the sliding bearing, the sintered layer may further contain at least one kind not more than 15 vol. % in total selected from the group consisting of particles hard in hardness and particles of a high melting point substance and may contain a solid lubricant not more than 10 vol. % which is dispersed in the sintered layer.
The sliding material may comprise a plated overlay layer bonded onto the sintered layer, the overlay layer being made of a Pbxe2x80x94Sn alloy containing at least one selected from the group consisting of In and Cu.